Question

Explain the low reactivity of haloarenes as compared to haloalkanes on the basis of resonance.

Answer 1

1. In haloarenes, the lone pair of electrons on the halogen atom becomes delocalized into the benzene ring through resonance. 
2. This delocalization results in the formation of multiple resonance structures, in which the carbon–halogen (C–X) bond acquires partial double bond character.
3. Due to this partial double bond character, the C–X bond becomes shorter and stronger, making it difficult to break during nucleophilic substitution.
4. Additionally, the electron delocalization reduces the positive character (electrophilicity) of the carbon atom bonded to the halogen.
5. As a result, the nucleophile finds it difficult to attack, and the nucleophilic substitution reaction proceeds very slowly in haloarenes.
6. In contrast, haloalkanes do not exhibit resonance.
7. Their C–X bond is a simple polar single bond, which is longer and weaker, and the carbon atom is more electrophilic.
8. This makes it easier for nucleophiles to attack, so haloalkanes undergo nucleophilic substitution reactions more readily.